The present invention relates to a waveform pre-equalization technique which improves digital signal fidelity, accuracy and recovery when recording/reproducing the digital signal onto/from a recording medium and, more particularly, to a pre-equalization technique used in a recording process, which tracks the record current overshoot occurring at a digital signal transition to an optimum record current level adjusted to the record head gap depth, to thereby adaptively adjust the amount of overshoot to the head gap depth. The record current is controlled by a record drive level signal. Recording compatibility between different gap depth heads when performing, for example, data append or data insert edit processes and the like, thus is achieved when the record current level is optimized.
Equalization techniques performed during reproduction and demodulation processes provide waveform compensation, whereby the equalized amplitude and shape of a digital data signal, comprising a plurality of consecutive isolated waves or digital data bits, is more nearly equal to the amplitude of the digital signal prior to the previous modulation and recording processes. However, digital signal recordings made by record heads of different gap depths, that is, new heads with deep gap depths or old heads with shallow gap depths, exhibit different magnetization patterns representative of the data bits when the signal is recorded on the magnetic medium. In the subsequent reproduction process, the different magnetization patterns usually require different equalizer settings to provide optimum data recovery. The different magnetization patterns also may cause recording compatibility problems in the event that recording media and recorder apparatus are interchanged in subsequent reproduction processes. For example, when reproducing digital data from a recording medium after a data append process in, for example, a data storage system, or after a data insert edit process in a video recording system, wherein the processes are done by different gap depth heads, the bit error rate may increase dramatically due to the sudden change in the reproduced waveform at the point of the append or insert edit. The sudden increase in bit error rate is due to the fact that a new equalizer setting for the appended or inserted data cannot be automatically adjusted rapidly enough from the prior equalizer setting to prevent the burst of bit errors.
In addition, overwriting deep gap depth recordings with a shallow gap depth head, or vice versa, as when performing an append or insert edit process, may also result in bit error rate degradation due to the incompatible recording depths in the medium.
In the investigation of recording head efficiency it has been found that a deep gap head generally has a lower efficiency and a faster high frequency efficiency roll-off when compared to the efficiency and roll-off of a shallow gap head. This difference in efficiency results in the inconsistent record process transfer functions and tape magnetization patterns of previous mention. It also has been found that, in order to perform a more efficient recording process, a deep gap depth head requires a greater amount of record drive current, and an increase in the amount of overshoot in the digital waveform at each transition. As applied herein, "overshoot" is that portion of a squarewave waveform which rises above the "flattop" of the waveform, and then decreases to the flattop value, and is expressed in percent of level increase over the flattop value. As the head wears and the depth of the gap decreases, successively less record drive current and overshoot are required to maintain the same strength and switching time of the head magnetic field and to provide a constant recorded magnetization depth profile in the recording medium. This in turn allows subsequently maintaining an optimum and constant equalizer setting corresponding to a minimum error rate during the reproduction process.
In the past, prior art recording/reproducing systems employing pre-equalization recording processes and equalization reproducing processes, purposely attempted to record perfect square waves representing the digital data. To this end, the overshoot amount of the pre-equalization was kept fixed for all heads. The typical record amplifier circuit included a resistor trim pot connected in parallel across the primary of the record transformer, whereby the resistance was pre-adjusted to cause the record output stage to produce square waves with some fixed value of overshoot at each transition. However, as discussed above, it has been found that contrary to typical practice, it is desirable to produce an automatically adjusted overshoot when recording with different record heads with varying gap depths to compensate for the varying high frequency characteristics of such heads.
It follows that it would be highly desirable to provide means for shaping the record current waveform to compensate for the inherent head efficiency changes due to different head gap depths, whereby the recordings made in media by various different gap depth heads automatically are made equivalent and compatible.